Exactly. I didn't know about the jessie-backports repo until yesterday.
I did add a .list and therefore are now able to install backport packages.

The thing is: If I want to install certbot, it tells me it depends on python-certbot. If I want to install that, it tells me it depends on python-acme, python-cryptography and python-openssl.....
Is this an endless thing? I read somewhere that it's not advised to install too many packages from the backports, but only those necessary... therefore I would like to keep the list rather short (also I have had experiences breaking my whole system by installing a new package.. so, minimal installation seems the safe route for me).

I tried with the command apt-get install certbot -t jessie-backports, is this limiting the install to this specific package? (Is apt-get normally supposed to download dependencies with the package it should install or not? If not, I have to get to the bottom of this myself, right?)

Is this an endless thing? I read somewhere that it's not advised to install too many packages from the backports, but only those necessary... therefore I would like to keep the list rather short (also I have had experiences breaking my whole system by installing a new package.. so, minimal installation seems the safe route for me).

Nope, it isn't endless and this is why omv-extras pins the proper packages (see here). If you want letsencrypt, there is no way around it with jessie/OMV 3.x. Most of the time, installing python packages (not python itself) from backports is minimal risk. Make a backup before doing this though...

I tried with the command apt-get install certbot -t jessie-backports, is this limiting the install to this specific package? (Is apt-get normally supposed to download dependencies with the package it should install or not? If not, I have to get to the bottom of this myself, right?)

Once the backports repo is enabled, do an apt clean from the web interface and you should be able to install the plugin from the web interface.

Hey there,
I just found out that my OMV does not generate the certificate for a second given domain.
In the plugin section I defined: "a.mydomain.com,b.mydomain.com" (without quotes) as domains.
After Pressing "Generate Certificates" I only receive a.mydomain.com in my "live" folder from LetsEncrypt.

Do you know this issue?

Best regards
Benedikt

I've got exactly he same issue. read above. Really wants a second certifite for my second domain. More people?

Once the backports repo is enabled, do an apt clean from the web interface and you should be able to install the plugin from the web interface.

I am not sure if it is... is adding the deb for the backports as a .list all I have to do to enable the repo?
because I added it to the omv-extra.list and the install from the plugin web interface still gives me the same error after the apt clean.

I guess I didn't realize this was an armhf system. That usually makes things a pain in the ass since they don't always have the same packages as i386/amd64 (especially in backports). What is the output of: